X-ray detectors are used in many applications including computed tomography (CT). Additionally, X-ray detectors are used in various other projective measurements, such as radiographic and fluoroscopic imaging that lie outside the scope of CT imaging.
CT scanners generally create images of one or more sectional slices through a subject's body. A radiation source, such as an X-ray source, irradiates the body from one side. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region (or a volume, for example, in cone-beam CT) defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
Historically, energy-integrating detectors have been used to measure CT projection data. More recently, photon-counting detectors (PCDs) have become a feasible alternative to conventional energy-integrating detectors. PCDs have many advantages including their capacity for performing spectral CT. To obtain the spectral nature of the transmitted X-ray data, the PCDs differentiate and record the incident X-ray photons using energy/spectrum bins, and count a number of photons in each energy/spectrum bin of each detector element.
Many clinical applications can benefit from spectral CT technology, which can provide improvement in material differentiation and beam-hardening correction. Further, semiconductor-based PCDs are a promising candidate for spectral CT, which is capable of providing better spectral information compared with conventional spectral CT technology (e.g., dual-source, kVp-switching, etc.).
Semiconductor-based PCDs used in spectral CT can detect incident photons and measure photon energy for every event. However, various complications arise due to phenomena such as pile-up, K-escape, and energy sharing. By accounting and correcting for these phenomena, improved projection data can be generated, resulting in higher image quality for CT image reconstruction.
Regarding fluorescence X-ray escape, when high energy photons impinge on a detector, the inner shell electrons from atoms of the detector are ejected from the atom as “photoelectrons.” After the ionization or excitation, the atom is in an excited state with a vacancy (hole) in the inner electron shell. Outer shell electrons then fall into the created holes, thereby emitting photons with energy equal to the energy difference between the two states. Since each element has a unique set of energy levels, each element emits a pattern of fluorescence X-rays that are characteristic of the element, termed “characteristic X-rays” or “fluorescence X-rays.” The intensity of the X-rays increases with the concentration of the corresponding element.
In many materials, such as Cadmium Telluride (CdTe) or Cadmium Zinc Telluride (CZT), the fluorescence X-rays primarily involve K-shell (closest shell to the nucleus of an atom) electrons. If the fluorescence X-rays escape from the detector, the detector signal is incorrect and the loss of energy incurred manifests itself as errors in the output spectrum of the detectors. Thus, the measured spectral signal can be distorted and may cause artifacts in the reconstructed image.
Regarding charge-sharing events, charge sharing occurs when charges from a detection event in one detector element or near the boundary between two adjacent detector elements results in the diffusion and migration from the point of detection to the electrodes of more than one electrode. Thus, a single detection event can result in electrical signals in more than one detector element, which can appear like two lower-energy detection events rather than a single higher-energy detection event.
Uncorrected, each of the above deviations from the ideal detector response can distort the detected spectrum relative to the incident spectrum, and can ultimately degrade the quality of reconstructed images and the material decomposition derived from the data.